In forming pipelines such as conventional gas and oil pipelines, successive lengths or sections of steel pipe are ordinarily welded together in end-to-end relation. The pipe ends are normally outwardly beveled to conjointly form a V-shaped annular groove or welding gap around the end-to-end arranged pipe sections, which gap is then filled with layers of weld metal to form a welded girth joint between the pipe sections.
The joining together of the pipe sections of pipelines by external welded girth joints is presently conventionally performed by automatic pipe welding apparatus such as, for example, that commercially known as the CRC M-200 Automatic Pipeline Welding System made by CRC-Evans Automatic Welding of Houston, Tex., and as disclosed, for instance, in U.S. Pat. No. 3,806,694 to Nelson et al. With this system, a consumable wire electrode is fed from the nozzle tip end of a welding head carried by a self-propelled carriage or so-called buggy which is orbited, during the welding operation, at a controlled speed around the end-to-end arranged pipe sections along a pipe encircling, endless metal band or track secured to one of the pipe sections adjacent the joint to be welded.
In the case where the pipe sections are to be welded together while in horizontally extending position, the wire feeding welding head of the welding apparatus is initially located at the top or twelve o'clock position of the welding gap or channelway between the two end-to-end arranged pipe sections and, during the continuance of the orbital welding pass of the apparatus around the pipe sections, is carried at a controlled speed along the track to the bottom or six o'clock position of the welding gap to form therein a layer or bead of weld metal extending half way around the welding gap between the two adjoining pipe sections. A second welding head carriage or buggy, which is reverse handed from the first one, is then mounted on the track at a top position thereon and moved therealong through a welding pass in the other direction around the welding gap to form a layer or bead of weld metal extending around the other half of the welding gap between the adjoining pipe sections and complete a full pipe encircling weld bead. Successive welding passes of the welding head carriages are then made in their respective directions from top to bottom along the carriage track to lay down successive layers of weld metal sufficient to fill the welding gap and form a composite final weld bead completely therearound constituting the welded girth joint of the pipe sections.
To assure the production of proper weld bead shape and placement, which is important for required X-ray quality and grain refinement to produce needed mechanical properties, the welding head and associated wire electrode are maintained, at the outset of each pass of the respective welding carriages, at a lead or drag angle of around 15.degree. or so to and in the plane of the weld joint, with the wire electrode tilted forwardly in the direction of movement thereof. At approximately the four-o'clock position of the welding head and wire electrode during each downward welding pass of the welding carriage, corresponding to approximately 120.degree. of movement of the welding head around the weld gap, the welding angle of the welding head and its protruding wire electrode is then changed to an approximately 15.degree. push or lag angle to the weld gap, with the wire electrode and welding head tilted rearwardly of the direction of movement of the welding head. The drag angle of the electrode wire on the upper part of the pipe, i.e. from the twelve o'clock to the four o'clock or 120.degree. position, holds back the weld metal and slag, resulting in optimum weld bead shape throughout this upper portion of the weld gap. The following push angle of the wire electrode on the bottom part of the pipe, i.e. from the four o'clock or 120.degree. position to the six o'clock or 180.degree. position, spreads the arc, controls penetration, reduces the puddle temperature and provides better weld bead shape throughout the lower portion of the weld gap. Without the proper wire electrode angle, the correct bead shape and placement cannot be achieved, and the result will be poor X-ray quality and inconsistent mechanical properties. It is important that this change of angle of the welding head and its protruding wire electrode be made with no change in position of the arc or variation in the travel speed, in order to deposit a uniform weld bead with no change in cross-section or shape.
Heretofore, so far as known, the welding angle of the welding head and nozzle of prior automatic pipeline welders for making welded girth pipe joints could be changed only by stopping the welding operation at the four o'clock or 120.degree. position of the welding head during each welding pass thereof around the pipe joint, unloosening the fastening of the welding head to the carriage frame, readjusting and resetting the welding head to the desired new welding angle position on the carriage frame, and then refastening the welding head in the reset new welding angle position on the carriage frame for the restart and continuance of the welding pass through the remaining portion thereof. This interruption of the welding operation, during the successive welding passes of the welding head and manual resetting of the welding angle position of the welding head, not only prolonged the time required for completing the welded girth pipe joint but also resulted in poorly formed joints having non-uniform weld beads of poor quality and placement in the weld gap.